Properties of soils Agriculture 2

What is the difference between dirt & soil?

How does soil help plants to grow?

Soil formation

Dirt vs soil Most people take soil for granted We call soil “dirt” and wash it off our clothes and our bodies. Dirt is soil out of place. Soil is essential to our survival and that of nearly every organism on earth. Our planet is mostly made of rock with an iron-nickel core.

Soils vary Scientists recognize five main factors that influence soil formation. 1. Parent Material 2. Climate 3. Living Organisms 4. Topography 5. Time

Parent Material Refers to both the organic and mineral material in which soil formation takes place. Mineral can include: Weathered rock, ash from volcanos and sediments deposited by wind and water. Soil formation will happen more quickly in material that are more permeable to water.

Climate Climate influences the amount of water available for weathering the parent material and the temperature at which it occurs. A warm, moist climate fosters plant growth and speeds up decomposition, both of which contribute to faster soil formation

Living organisms Plants supply soil with organic material and help prevent erosion. Deep-rooted plants have a greater impact on soil formation than shallow-rooted plants because they create larger channels for water movement. Insects, earthworms, fungi and bacteria are important because they help decompose organic material releasing plant nutrients. EROSION: is the action of exogenic (water flow or wind) processes which remove soil and rock from one location on the earths crust, then transport it to another location where it is deposited.

Topography The three dimensional shape of the land influences water movement and therefore the speed of soil formation. Since water flows downward due to gravity, soils on slopes are prone to erosion. Areas that are very wet or very dry may not be fertile and the resulting lack of plant growth can slow the rate of soil formation.

Time The weathering of rock slowly produces soils. Constant exposure to wind and rain cause the rocky crust to break slowly down into smaller particles. It can take centuries to produce fertile topsoil. As rainwater seeps into cracks, temperature extremes cause the water to freeze. The rock expands, contracts and fractures. Organisms that live on and in the soil help these weathering actions along.

Humans Of course, in addition to these five factors, human activity also can influence soil formation. Agricultural practices and urban development especially can interfere with the naturally occurring process of soil formation.

Soil horizons

Horizontal layers The gradual process of soil formation produces a series of horizontal layers. A SOIL HORIZON: is a layer generally parallel to the soil surface, whose physical and chemical characteristics differ from the layers above and beneath. A given type of soil usually has three or four horizons. Horizons are characterized by the obvious physical characteristics of color and texture. The formation of soil horizons is influenced by factors such as air, water, sunlight and plant material. The weathering of parent material occurs first at the surface and then works its way downward. The uppermost layers are changed (weathered) the most, while the deepest layers are the most similar to the original parent material

Soil Horizons

O-a-b-c-r The major components of soil (in addition to the organic matter) are sand, silt and clay. Each horizon is labeled with its own capital letter that identifies its place. O: Refers to organic matter that is relatively undecomposed and lies on the surface A: Refers to organic material mixed with mineral that makes up the surface soil. B: Refers to the subsoil. This layer reflects the chemical and physical alteration of the original parent material. C: Refers to the parent material R: Refers to the bedrock. The R layer is mostly composed of hard rock that cannot be excavated by hand.

Particle size

Sand, Silt & Clay

Particle size Gravel: larger than 2mm; feels coarse Sand: 2 - .05mm; feels gritty Silt: .05 - .002mm; feels like flour Clay: smaller than .002mm; feels sticky when wet Every soil type is a mixture of sand, silt and clay.

Sand Too Coarse Soils with lots of sand have big spaces between the particles. They don’t hold water or nutrients. Sandy soils don’t stick together very well. Plant roots cant hold onto this soil. But the big spaces do allow air into the soil. There are some plants that are able to grow in sandy topsoil by putting their roots deep, through the sand to the subsoil.

Silt Too light This is material which is finer than sand, but still feels gritty. Silt is commonly found in floodplains and is the soil component that makes mud. Soils with a lot of silt make excellent farm land, but erode easily. This is the soil blown away in dust storms and carried down stream in floods.

clay Too fine Lots of clay makes the soil heavy and dense. The spaces between soil particles are very tiny. When clay soil is dry, its almost as hard as concrete. Plant roots cant push through it. No air can get in from the surface. Most bacteria and other soil organisms that need oxygen cant breathe. But clay is important because it can change the soil chemistry.

Loam Just Right The perfect soil for plants and soil organisms has about the same amount of sand and silt plus a smaller amount of clay. This soil has enough large and small spaces for air and water to flow in. It also has enough clay to let it stick together and hold humus. The dark organic material in soils, produced by the decomposition of vegetable or animal matter and is essential to the fertility of the earth. These clumps make another size of space. Plant roots can easily grow through these spaces.

Particle surface area

How does the surface area of a large particle (sand) compare with the surface area of a small particle (clay)

Surface area & particle size Calculating surface area of a cube is simpler than calculating surface area of a sphere. Surface area = Length X Width X Number of sides

Soil triangle

Soil texture During soil formation, inorganic material is broken down by weathering into particles of various sizes. SOIL TEXTURE: refers to the relative proportions of different-sized particles found in the soil. Scientists classify soil particles into three categories. Smallest particles: measure less than .002 mm – called clay The next-largest particles: measure from .002 - .06 mm - are called silt Largest particles: .06 – 2 mm – Sand

Soil triangle Soils vary in their proportions of clay, silt and sand Soil scientists classify different soil types using the soil triangle. Each side of the soil triangle represents the amount of a particle of a certain size – clay, silt and sand. The relative amounts of these three soil components intersect within the triangle and determine to what type of soil those proportions correspond. The soil triangle is used to classify the different types of soil

Soil triangle

Soil triangle Soil triangle diagram is divided into 12 soil classifications based on the amount of sand, silt and clay in the soil. A loam includes an ideal mix of sand, silt and clay for many crop plants.

Classify soils Using your soil triangle, how would you classify a soil that contains 20 % Clay, 70% Silt and 10% sand? Start by finding 20% clay on the left side and drawing a line horizontally across the triangle. Then find 70 % silt on the right side of the triangle and draw a ling for that percentage. Finally draw a line for 10% sand from the bottom of the triangle. The intersection of these 3 lines will determine the soil classification. -Silt Loam

In which category on the soil triangle do you think crop plants would grow best and why?

Properties of soil particles The ability of a soil to accept and retain water is largely determined by the relative amounts of clay, silt and sand present. POROSITY: Refers to spaces in the soil that can hold either air or water. PERMEABILITY: Is defined as the rate at which water can travel through soil Loamy soils typically contain about 50% air space, which allows root systems to “breathe” The organic material is composed of living organisms, plant roots and plant and animal residue. 1 gram of healthy topsoil contains: 100 nematodes, 1 million fungi and 1 billion bacteria